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Unit-4 Current and Potential TransformerQ1)Explain the construction of current transformer?A1) The core of the current transformer is made of lamination of silicon steel. For high degree of accuracy, the Permalloy or Mumetal is used for making cores. The primary windings of the current transformers carry the current, which is to be measured, connected to the main circuit. The secondary windings of the transformer carry the current proportional to the current to be measured, connected to the current windings of the meters or the instruments.The primary and the secondary windings are insulated from the cores and each other. The primary winding is a single turn winding called bar primary and carries the full load current. The secondary winding of the transformers has many turns.

Figure . Current Transformer ConstructionQ2)Explain the construction of potential transformer?A2) The potential transformer is connected in parallel with the circuit. The primary windings of the potential transformer are directly connected to the power circuit where voltage is measured. The secondary terminals of the potential transformer are connected to the measuring instrument like the voltmeter, wattmeter, etc.The secondary windings of the potential transformer are magnetically coupled through the magnetic circuit of the primary windings.The primary terminal of the transformer is rated for 400V to several thousand volts, and the secondary terminal is always rated for 400V. The ratio of the primary voltage to the secondary voltage is termed as transformation ratio or turn ratio.Q3)Explain the characteristic of current transformer?A3) Current Ratio – known as the Turns Ratio is the ratio of the primary current to the secondary current. It is equal to the ratio of number of turns in the primary and secondary coils. Is = Ip(Np/Ns) Ip= Secondary current. Np=Primary coil turns Ip: Primary current Ns: Seondary coil turns Polarity – The polarity of a CT is determined by the direction of coil winding around the core of the CT either clockwise or counter-clockwise and by the way the leads, if any, are brought out of the transformer case. All current transformers have subtractive polarity. Accuracy Class – Accuracy Class describes the performance characteristics of a Current Transformer and the maximum burden that can be allowed on its secondary circuitMetering accuracy CT can deliver high accuracy current measurement over short ranges of current.Q4)Explain the working of potential transformer?A4) The working of a potential transformer is that it steps down the high voltage and provide the power according to the reading of a measuring instrument which is installed for the measurement of voltage or power. So, its main purpose is to provide voltage according to the measuring instrument so that the power is controlled according to its rating.

Figure. Equivalent circuit of potential transformerVp = primary voltageEp = induced voltage in primary windingVs = secondary voltageEs = induced voltage in secondary windingIp = primary currentIs = secondary currentI0 = no load currentIc = core loss component of currentIm = magnetizing component of currentRp = resistance of primary windingXp = reactance of primary windingRs = secondary winding resistanceXs = secondary winding reactanceRc = imaginary resistance / core lossXm = magnetizing reactanceRe = resistance of external loadXe = reactance of external loadNp = primary winding turnsNs = secondary winding turnsN = turn ratioΦ = flux of potential transformerδ = Phase angle between secondary winding voltage and secondary winding β = Phase angle between primary current and Secondary currentα = Phase angle between no-load current I0 and flux φ

Figure . Phasor Diagram of PTThe flux is conspired along the x axis. Im is in phase with flux. Ic leads by Im 900. The sum of Ic and Im produces no load current Io. Ep is in the phase with the core loss component of current Ic. Es is 180o out of phase with the primary winding voltage Ep. Secondary voltage Vs are obtained by subtracting the IsRs and IsXs from secondary voltage.Q5)Briefly describe the working of AC voltmeters using Rectifiers?A5) The rectifier is used to convert a.c. voltage to be measured, to d.c. This d.c., if required is amplified and then given to the PMMC movement.The r.m.s means root-mean square i.e. squaring, finding the mean i.e. average and finally root. If the waveform is continuous then instead of squaring and calculating mean, the integration is used. Mathematically the r.m.s. value of the continuous a.c. voltage having time period T is given by,Vrms =

1/T

2 dt -------------------------------------(1)Here 1/T term indicates the mean value or average value. For purely sinusoidal quantity Vrms = 0.707 Vm where Vm= peak value of the sinusoidal quantity.Vav = 2/T

dtThe interval T/2 indicates the average over half a cycle. For purely sinusoidal quantity Vav = 2/ π Vm = 0.636 Vm Where Vm = Peak value of sinusoidal quantity. The form factor is given by Kf = r.m.s value/ average value = form factor. Key Take Aways:Rectifier type instruments generally use a PMMC movement along with a rectifier arrangement. Silicon diodes are preferred because of their low reverse current and high forward current ratings. Q6)Explain the working of true rms voltmeter?A6)

Figure. True RMS VoltmeterThere are two thermocouples namely measuring thermocouple and balancing thermocouple. They form part of bridge at the input of DC amplifier. The same non- linearity effect takes place in both the thermocouple, so this effect is compensated. An input AC voltage to be measured is first amplified by using AC amplifier, thus the smaller input voltages can be measured. Coupling capacitor is used at input to avoid any DC component in the measurement. The output of AC amplifier is connected to the heating element of measuring thermocouple. This will generate the heating wave at this thermocouple. Now this thermocouple will generate the voltage proportional to the heating wave. This voltage generated by measuring thermocouple makes the bridge unbalanced. This voltage is amplified by the DC amplifier.The output current form DC amplifier is acting as a feedback current. This creates heating of balancing thermocouple. The balancing thermocouple gives the voltages proportional to the heating wave. A stage will be reached at which output of measuring thermocouple becomes equal to the output of balancing thermocouple. At this point, the balancing of bridge circuit takes place. So, the feedback current becomes equal to an input current. This current is now used to cause the movement of indicating meter. Q7)Explain the working principle of Q meter?A7) A device that is used to measure the QF or storage factor or quality factor of the circuit at radio frequencies is called the Q-meter.

Figure11. Q-meter Oscilloscope In the circuit, the energy of the oscillator can be supplied to the tank circuit. This circuit may be adjusted for the resonance through unstable ‘C’ until the voltmeter reads the utmost value.The o/p voltage of resonance is ‘E’, equivalent to ‘Ec’ is E = Q X e and Q = E/e. Because ‘e’ is known so the voltmeter is adjusted to read ‘Q’ value directly.The coil is connected to the two test terminals of the instrument to determine the coil’s inductanceThis circuit is adjusted to resonance through changing the oscillator frequency otherwise the capacitance. Once the capacitance is changed, then the frequency of the oscillator can be adjusted to a specified frequency & resonance is attained.If the value of capacitance is already fixed to a preferred value, then the frequency of the oscillator will be changed until resonance takes place.The reading of ‘Q’ on the o/p meter is multiplied through the setting of an index to get the actual ‘Q’ value. The coil’s inductance is calculated from known values of the coil frequency as well as the resonating capacitor.Q8)Explain delay line oscilloscope?A8)

Figure. Delay Line in OscilloscopeA delay is caused by the circuit in transmitting the sweep signal to the horizontal deflecting plates. The delay that is caused by the horizontal deflecting circuits is about 80 ns. To present the leading edge of the signal under observation, the signal to the vertical deflecting plates must be delayed by the same time at least. The delay line provides the required delay to the vertical deflecting voltage. Delay line can theoretically be inserted at any place in the vertical deflecting system. There are two types of delay lines used with CROs. The lumped parameter delay line and the distributed parameter delay line.Q9) Explain the working of multiple trace oscilloscope?A9) In this method, the same electron beam is used to generate two traces which can be deflected from two independent vertical sources. They are used to generate two independent traces which the alternate sweep method and other is chop method. The block diagram of dual trace oscilloscope is shown in the Figure. There are two separate vertical input channels A and B. A separate preamplifier and - attenuator stage exists for each channel. Hence amplitude of each input can be individually controlled. After preamplifier stage, both the signals are fed to an electronic switch. The switch has an ability to pass one channel at a time via delay line to the vertical amplifier. The time base circuit uses a trigger selector switch 52 which allows the circuit to be triggered on either A or B channel, on- line frequency or on an external signal. The horizontal amplifier is fed from the sweep generator or the B channel via switch 5! and 51. The XY mode means, the oscilloscope operates from channel A as the vertical signal and the channel B as the horizontal signal. Thus, in this mode accurate X-Y measurements can be done.

Figure. Multiple trace OscilloscopeQ10) Explain the working of Digital oscilloscopes?A10) The digital oscilloscope digitises and stores the input signal. This is done using CRT and digital memory. The block diagram digital oscilloscope is shown in the figure below. The digitisation is made by taking the sample input signals at periodic waveforms.

Figure 14. Digital Storage OscilloscopeThe maximum frequency of the signal is measured by the digital oscilloscope depending on the two factors. They are

Sampling rate

Nature of converter.

Sampling Rate – The sampling theory states that the sampling rate of the signal must be twice as fast as the highest frequency of the input signal. The sampling rate means analogue to digital converter has a high fast conversion rate.Converter – The converter uses the expensive flash whose resolution decreases with the increases of a sampling rate. Because of the sampling rate, the bandwidth and resolution of the oscilloscope are limited.The need of the analogue to digital signal converters can also be overcome by using the shift register.

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